1. Field of the Invention
The present invention relates generally to improved methods and compositions for cementing pipe in well bores.
2. Description of the Prior Art
Hydraulic cement compositions are commonly utilized in primary sealing operations whereby strings of pipe such as casing and liners are sealed in well bores. In performing primary cementing, a hydraulic cement composition is pumped into the annular space between the walls of the well bore and the exterior surfaces of the pipe disposed therein. The cement composition is permitted to set in the annular space thereby forming an annular sheath of hardened substantially impermeable cement therein. The sheath physically supports and positions the pipe in the well bore and is intended to bond the exterior surfaces of the pipe to the walls of the well bore whereby the undesirable migration of fluids between zones or formations penetrated by the well bore is prevented.
In a deep, high temperature well bore penetrating weak formations, the cementing of a pipe string in the well bore is often difficult and characterized by lost circulation during cementing and gas migration through the resulting cement sheath. These problems are principally caused by the use of a cement slurry having insufficient fluid loss control and using an aqueous spacer between the drilling fluid and the cement slurry which does not adequately remove drilling fluid from the pipe surfaces and the walls of the well bore. Further, at the high temperatures encountered, when an oil based drilling fluid is used and the aqueous spacer mixes with oil on the walls of the well bore, solidification of the mixture often results. The presence of drilling fluid and/or solidified oil-water mixtures in the well bore prevents the cement slurry from adequately bonding to the pipe and walls of the well bore, and as a consequence, costly remedial cementing procedures are required.
Thus, there are needs for improved methods and compositions for cementing pipe strings in well bores drilled using oil based drilling fluids.